Bone structures are typically comprised of two types of bone, cortical bone and cancellous bone. Cortical bone can be characterized as a rigid and dense material, whereas cancellous bone can be characterized as a structured material with a high degree of visible porosity. Cortical bone and cancellous bone combine to form structures that are strong and lightweight. However, strength can be compromised by osteoporosis, a metabolic disease characterized by a decrease in bone mass. It is estimated that osteoporosis affects approximately 15-20 million people in the United States. Although osteoporosis can affect persons of all ages and both genders, it is generally a disease associated with the elderly. Approximately 1.3 million new fractures each year are associated with osteoporosis, and the most common fracture sites are the hip, wrist and vertebrae. Osteoporosis leads to skeletal fractures under light to moderate trauma and, in its advanced state, can lead to fractures under normal physiologic loading conditions.
Whether treating a fracture associated with osteoporosis or another disorder of the musculoskeletal system, implant attachment to weakened osteoporotic bone can be problematic. Inadequate attachment of an orthopaedic implant to osteoporotic bone can result in less effective, or ineffective implant fixation.
Generally, orthopaedic implant fixation is accomplished by numerous conventional attachment mechanisms to include screw thread, serration, spikes, barbs, porous coatings, and treated surfaces. Some attachment mechanisms expand within bone, analogous to a rivet or wall anchor. Bone cements are also used for orthopaedic implant fixation, primarily as an adhesive interface layer between implant and bone. Bone cement can also be used to augment cancellous bone adjacent to an attachment region, wherein bone cement is used to fill or partially fill cancellous bone pores.
A common approach to implant fixation is the screw thread, used on implants such as bone screws. Bone screws can be used as stand-alone devices for attaching fractured bone or used in a multi-component implant assembly. Tightening bone screws is generally subjective and the appropriate fixation is especially difficult to judge when securing a bone screw to osteoporotic cancellous bone. Over-tightening can lead to stripping of bone and inadequate fixation, while under-tightening can also lead to inadequate fixation. After a screw has failed to hold, bone cement can be used to augment screw fixation by filling a drilled hole with the bone cement, or by coating the screw thread with bone cement prior to reinsertion. These time consuming repair techniques have experienced some success; however, the necessity for repair emphasizes the potential ineffectiveness of screw thread purchase in osteoporotic bone. Also, bone screws can occasionally loosen, losing their effectiveness. Further, loose bone screws can ultimately back-out and migrate to an undesirable position or location.
There are numerous examples of orthopaedic implants that serve as conduits for the delivery of synthetic material, usually bone cement, to specific bone/implant interface regions. Cannulated bone screws adapted with screw thread apertures for the delivery of bone cement are described in U.S. Pat. No. 4,653,489 to Tronzo, U.S. Pat. No. 6,048,343 to Mathis et. al., U.S. Pat. No. 6,210,376 to Grayson, and U.S. Pat. No. 6,214,012, to Karpmen et. al. Foremost, the addition of apertures to a screw thread substantially weakens the bone screw. Another disadvantage is the potential for uneven distribution of bone cement within cancellous bone, caused in part by bone pore regions not directly adjacent to apertures receiving a disproportionate amount of the injected cement. In addition, extruding directly into bone can require relatively high pressures depending on the bone characteristics and the viscosity of the injectable material. Injection at lower pressure is preferred because simpler injection systems can be used and migration of injectable material is less likely.
There are known concepts of non-threaded orthopaedic implants serving as conduits for the delivery of bone cement, or other materials for implant fixation. Examples include implant fixation within an intramedullary canal, such as an intramedullary nail used for fracture fixation. For example, U.S. Pat. No. 4,369,772 to Miller describes a method for strengthening a fractured femur which comprises drilling a hole along the axis of the medullary canal of the bone, inserting in the hole a substantially inflexible tube having an outside diameter less than the diameter of the hole, injecting into the tube and around the tube a semisolid hardenable bone cement, and allowing time for the mixture to harden. U.S. Pat. No. 5,514,137 to Coutts describes a cannulated intramedullary nail adapted for the extrusion of resorbable bone cement from the distal tip in order to augment cancellous bone in the distal region of the nail.
Another mechanism for attaching an implant to bone is disclosed in U.S. Pat. No. 4,065,817 to Branemark. The implant described in the patent to Branemark is formed as a tubular support member having perforations therein, the end of the bone is bored, the tubular member is introduced into the bore and cement is introduced into the interior of the tubular support and passes out through the perforations to provide the midterm anchor on the walls of the bone.
A need exists to develop improved implant fixation to bone, and in particular, implant fixation to osteoporotic bone. Preferably, inventions to improve implant fixation to bone should be applicable to a wide range of implant systems, and also be readily adaptable to minimally invasive surgical techniques.